Landing an aircraft requires intense concentration to establish proper coordination of the aircraft controls. One of the most important aspects of landing an aircraft is to ensure that the aircraft is properly aligned with the landing area. This typically involves two separate activities. One activity is to assure that the aircraft is landing over the centerline of the landing area. The other activity is to assure that the aircraft is approaching the landing area at the proper speed and vertical descent angle to prevent stalling the aircraft while still landing at a low velocity.
When the landing area is in sight, an aircraft pilot can generally use visual cues to align the aircraft along the centerline of the landing area and at the right descent angle. Although landing an aircraft visually in daylight is difficult, it is even more difficult at night, when the pilot has fewer visual cues available.
A pilot landing an aircraft at a landing field having conventional visual aids typically flies into a glideslope pattern and follows the glideslope while attempting to maneuver over the centerline extension of the landing area. When a pilot is maneuvering into the glideslope pattern from the side of the landing area, it is somewhat difficult to coordinate the turns so that the aircraft is approximately aligned with the centerline extension when the aircraft intercepts the glideslope. Accordingly, it is desirable to have a method and apparatus for landing an aircraft at a landing area which gives highly accurate cues to the position of an aircraft relative to the landing area's centerline and/or desired glideslope.
One particularly severe landing task is landing an aircraft on a carrier, especially aligning the aircraft with the carrier's landing area centerline. Smaller aircraft landing on carriers must be within twenty feet of the centerline, while the margin is ten feet for aircraft with greater wingspans. Complicating the task of landing an aircraft on a carrier is the fact that modern carriers have a landing deck angled 10 degrees from the ship's centerline. Due to the carrier's motion through the water, the ship's centerline typically moves to the right at about 10 knots.
During the day there are many visual cues that the pilot can use when landing on a carrier. However, during the night the visual cues are largely missing. Only the angled deck is lighted. Nighttime loss of depth perception further increases the pilot's difficulties.
Even on the largest carriers, the angled deck measures only 786 feet by 100 feet. The actual landing area is 120 feet long along the deck by 40 feet wide across the deck. The landing area includes four arresting wires spaced 40 feet apart. Thus, if the pilot is 20 feet above the optimum glideslope, the aircraft will miss the arresting wires and have to make a touch-and-go. If the pilot is 20 feet below the optimum glideslope, the aircraft will crash into the stern end of the carrier.
A landing aircraft approaches a carrier at speeds between 105 and 135 knots. Due to these high approach speeds, high momentum, and relatively slow control response of jet aircraft in the landing configuration, it is possible for a small initial drift of the aircraft from the centerline and glideslope to become a serious and potentially dangerous misalignment to the arresting cables and landing area.
Visual aids have been developed to assist in landing aircraft, especially at night. Other than lighting systems that highlight the outline and centerline of the landing area, these visual systems have been developed to assist in the vertical descent guidance of the aircraft. There are no presently-installed precision centerline systems. The visual aids known in the prior art all require the pilot to exercise judgement, such as attempting to fly over a row of lights designating the centerline extension of the runway or keeping lights designating the runway's outline symmetric.
At present there are two primary vertical guidance systems. These systems are Visual Approach Slope Indicator (VASI) and Precision Approach Path Lighting (PAPI). Both of these systems use sets of red and white lights, placed to the side of the runway, which give patterns indicative of the aircraft's placement relative to the desired glideslope. The VASI system includes two sets of red and white lights, one set placed behind the other relative to the approach direction of an aircraft and to the left of the runway as seen from the landing aircraft. Both sets of lights produce adjacent angularly displaced red and white beams directed toward the landing aircraft. In both sets of lights, the upper segment is colored white and the lower segment is colored red. The elevation angles of the two sets of lights are arranged so that when an aircraft is flying on the desired glideslope, the pilot will see the red segment from the farther set of lights and the white segment from the nearer set of lights. If the aircraft is above the desired glideslope, the pilot will see the upper white segments of both sets of lights. If the aircraft is below the desired glideslope, the pilot will see the red segments of both sets of lights.
In the PAPI system, four sets of lights, each having an upper white segment and a lower red segment, as in VASI, are placed side-by-side to the left of the runway as seen by the landing aircraft. The four sets of lights are pointing toward the landing aircraft, but at different elevation angles. The left-hand set of lights has the highest elevation angle and the right-hand set of lights has the lowest elevation angle. The middle two sets of lights are arranged so that a pilot in an aircraft landing along the desired glideslope will see the red light from the two left-hand sets of lights and the white lights from the two right-hand sets of lights. If the aircraft falls below the desired glideslope, the pilot will see the third set of lights from the left turn from white to red, leaving three red lights and one white light. Falling even farther below the desired glideslope will cause all four sets of lights to turn red, indicating danger. On the other hand, if the aircraft rises above the glideslope, the pilot will see one red light and three white lights at first, and then four white lights if it rises sufficiently far.
Two difficulties with such incandescent visual aids, as well as aids using fluorescent or arc lights, are a lack of spatial coherence and a lack of spectral purity. These difficulties cause the transition of an aircraft between a white segment and a red segment to be somewhat muddled, because the lack of spatial coherence causes the boundaries of the two beams to be imprecisely defined. There is a significant period in this transition where the color from such a set of lights appears pink, rather than white or red. In addition, atmospheric scattering removes the shorter wavelength, i.e., bluer, light from the white segments of these sets of lights. This causes the white light itself to appear somewhat pinkish, even before it is mixed with the light produced by the red segment of each set. Accordingly, it is desirable to have visual aids that are not subject to these faults.
Carriers are presently equipped with the Fresnel lens optical landing system (FLOLS). FLOLS is stabilized to account for the carrier's motions and has a maximum range of about 3/4 mile. A pilot using FLOLS sees an amber ball (the "meatball") which is aligned with a row of horizontal green datum lights. When the aircraft is below the desired glideslope, the amber ball appears to be below the datum lights. If the aircraft is somewhat farther below the desired glideslope, the color of the ball changes from amber to red. Below this level, the ball disappears from the bottom of the FLOLS display. While optical glideslope landing systems that are more precise and better stabilized are available, even they are only useful out to a maximum range of 11/4 miles. It is desirable to have visual landing aids with a greater useful maximum range.
One way of overcoming many of the difficulties associated with present-day landing systems (especially on carriers) is to transmit laser light. Laser light has a high spatial coherence and great spectral purity. Because of its high spatial coherence, laser light can produce crisp displays which seem to come from a single point source and are easy to detect by peripheral vision. For example, the "fuzziness" associated with the edge of a laser-based display is only about one inch in width at a range of one mile. In addition, lasers can be used to produce more accurate light corridors (to the limits of diffraction), which have been found to be usable at ranges of at least twelve miles. This overcomes the requirement for a pilot to resolve visual aids before they become useful because the pilot needs only to recognize the colors of the light corridor. Further, laser systems are easy to align and the spectral purity of their light makes them easy to distinguish. Also, lasers presently have life expectancies of from 4,000 to 10,000 hours and produce an average luminous intensity of 500 candela, which provides for very long range acquisition of the laser signals. The laser's monochromaticity gives laser beams high color contrast with the surroundings and makes them easy to identify. Their identification is further enhanced by coherent effects which make the laser seem to have a "texture."
Lasers require only low input power. This, with their collimation, increases the covertness of visual landing aids based on lasers. Since the laser light comes from a virtual point source, small exit apertures and small, lightweight optical elements can be used in laser-based visual landing aids.